It is often necessary to be able to measure the location of a point with respect to one or more axes. For instance, in machine tool operation, it is necessary to locate the tool or the workpiece, both of which can be considered to be points, with respect to the stage of the machine, which can be considered to be defined by one, two or three axes, depending on the machine. As an example, with a lathe, the cutting tool, a point, must be located with respect to the three axes of the machine. Human to computer interfaces also must identify the location of a point with respect to an axis. The input device of a stylus and pad uses a pair of axes extended in two dimensions, defining the pad, and a reference point, the tip of the stylus. In this application, the point "moves" from the perspective of the user, and the pad remains fixed. The problem may also be conceptualized as measuring the position of an object, extended in space along an axis, with respect to a fixed point. The principal example discussed below entails an axis of changing location and a fixed reference point.
The present invention was developed during the course of analyzing the interface between humans and musical instruments, in particular a bowed string instrument such as the cello. It is desireable to be able to unobtrusively and precisely map various aspects of play, including the bow pressure, bow/wrist orientation, finger position on the strings and the position of the cellist's bow with respect to a fixed point on the cello, for instance the bridge. These are quantifiable aspects of the gestures that the cellist applies to the instrument. Knowing this information, the investigator can analyze the gestures that the cellist uses, and can map those gestures to the music that the cello produces in response. The information is used to understand the techniques of instrumentalists, both accomplished and beginners. The information is also used to generate a real-time performance on an electronic instrument, that recreates the sound of a cello and of other instruments, both real and electronic only.
The present invention takes advantage of the near field electromagnetic effect and thus can be used in instances where the range of motion to be studied is on the order of the same size as the antennae used to analyze the motion. Thus, appropriate candidates for such a method of study are those where an antenna, which is of the appropriate size, can be placed. For example, valuable information may be gained from mapping the swing of a golf club, baseball bat or tennis racquet. Each of the foregoing is an appropriate structure upon which to mount an antenna. In each case, a second antenna would be mounted at a known position, for instance on the athlete. Knowledge of the general motions of a person with respect to a computer input device also has wide applicability in the control of computers. The foregoing lists only some of the most obvious instances of application of the present invention.
To measure the position of an extended object with respect to a reference point, (or of a point with respect to an axis) it is necessary to be able to measure two distances for an object moving in the two dimensions of a plane and three distances (or two distances and an angle) for an object moving in three-dimensional space. Several techniques have been proposed, however, none are deemed to be adequate for a delicate application, such as measuring the position of a cellist's bow during play.
Acoustic sonar analysis, measuring the propagation time of an ultrasonic sound pulse in the air, requires bulky equipment. It is also very difficult to accomplish without coupling some energy into the audible range, which would interfere with the acoustical signal, i.e. the music, generated by the cello. Acoustic phase analysis, measuring the phase shift in a received audio signal, has the same problems as acoustic sonar analysis. Infrared strength analysis, measuring the fall-off in signal strength between an infrared diode and detector, can be done compactly. However, it requires maintaining a direct line-of-sight between the source and detector, and is sensitive to stray reflections. Due to the motion of the cellist's bow, particularly the rotation and rocking of the bow, a direct line-of-sight is not always available. Inductance proximity analysis, measuring the eddy-current coupling emerging from a coil, is limited to distances on the order of the size of the coil, typically, one centimeter, a scale that is too small for analysis of the position of a cello bow and many other human/instrument interface questions. Microwave reflectivity analysis, measuring the reflected microwave signal transmission time, requires a well-characterized target of stable geometry. The rotating and twisting motion of the cello bow renders it an unsuitable target for microwave radiation.
U.S. Pat. No. 4,980,519, issued to Max V. Mathews on Dec. 25, 1990 and assigned to the Trustees of Stanford University discloses an electronic "drum" that is excited by one or more batons. Each baton transmits a signal of a distinct radio frequency from a position in space. The x, y, and z position of the transmitter with respect to the "drum" can be determined. The drum has a flat surface, carrying at least two pairs of electrodes. Both pairs of electrodes are shaped so that the degree of capacitive coupling between each baton transmitter and the pairs of electrodes corresponds to the position of the transmitter in the x and y direction. The z position (distance away from the drum) is determined computationally.
The two pairs of electrodes on the drum must be positioned so that neither shields the other. It is also necessary that they be shaped so that the capacitance established between the baton and the electrodes varies with the location of the baton. The electrodes are made up of interlaced adjacent triangles, much like the pattern on a backgammon game board. One member of a pair of electrodes consists of several triangles of varying sizes, each having one short side and two longer sides. The short sides are aligned in line with each other, with the longer sides all extending away from the short side in the same direction. The other member of the electrode pair is similarly shaped, and is arranged with the short sides of the triangles parallel to the short sides of the first member, and with the longer sides pointing toward the first electrode, so that the points of the triangles interleave.
The variation in shape and size is important. Such a scheme is not feasible for measurement of the position of long slender extended objects, such as a cellist's bow, a conductor's baton or a golf club, because there is not enough surface area on the extended object upon which to locate an electrode of varying shape.
Another known apparatus for determining the position of a point relative to a reference uses two transmitting antennae and a single receiving antenna. This device takes advantage of the capacitive linkage between the transmitting antennae and the receiving antenna. By transmitting signals of two different frequencies, it is possible to determine the distance (in a plane) of the receiving antenna from each of the transmitting antennae, and thus its position. This technique does not work well for measuring the position with respect to an axis extending over a distance larger than the characteristic length of the capacitive effect. This length is on the order of the size of the antenna, typically 10-30 cm for study of human machine interfaces. Therefore, using only two antennae, it would not be practical to determine the position, in the direction of its length, of a twenty cm. long cello bow, moving through a range of twice its length, with respect to the bridge. To do so would require antennae too large to be conveniently applied to the cello. It would be necessary to locate a receiving antenna approximately every ten cm. along the length of the bow.
It will be understood that, while the following discussion addresses locating the position of the bow (the axis) with respect to the bridge (the point) a more general application is determining the location of a point with respect to one or more axes. However, it is more natural to speak of determining the location of the cello bow with respect to the bridge, since it is the bow that moves, relative to a human observer. For other applications, however, it is more natural to regard the point as moving with respect to one or more axes.